How Odense’s Water Resource Recovery Facility became a leading light in clean energy

How did Ejby Mølle gain energy independence so quickly?

The answer is straightforward: a combination of visionary leadership, technical expertise, global collaboration, and continuous innovation was central in converting an ageing treatment plant into a world leader in energy efficiency. The result is not just a better plant, but a redefined role for utilities in supporting local decarbonisation efforts.

A phased approach to self-sufficiency

However, the project’s journey was not always straightforward. From the outset, it faced significant challenges. Wastewater treatment is a high-cost, energy-intensive public service, accounting for more than 1% of total electricity consumption in Europe. WRRFs also emit high greenhouse gas levels, especially nitrous oxide, which is approximately 300 times more potent than carbon dioxide. Additionally, most utilities of Ejby Mølle’s size achieve only 40–50% energy self-sufficiency, with full energy positivity often taking decades and requiring major capital investment. For a utility serving a population of more than 400,000, any major upgrades also had to be made without service interruption, further complicating the task. As one of Denmark’s oldest facilities, Ejby Mølle’s infrastructure was ageing, and there were strict nutrient limits in its discharge into a local river. In response, VCS set out to redefine the parameters of energy efficiency projects, without compromising environmental compliance.

By 2012, the utility’s own measures saw Ejby Mølle attain 77% energy self-sufficiency. Many of these early improvements were low-cost and involved optimising existing systems, proving that innovation doesn’t always require large capital outlay. The phased approach gave VCS the flexibility to assess results after each stage and build support incrementally, both internally and with external stakeholders.

Partnering for powerful results

Achieving full energy positivity required the support of an outside partner. VCS reached out to Jacobs, on the back of a technical paper the solutions provider had developed about energy optimisation. Initially, the collaboration focused on the development of modelling and diagnostics that simulated every component of the plant – from nutrient removal and sludge processing to biogas generation – allowing efficient and thorough evaluation of alternative operating and facility upgrade scenarios.

The plant’s success is a reminder that innovation is most effective when implemented at the operational level, close to the challenge itself

The use of plant-wide modelling gave both parties the ability to test assumptions, run scenario planning and avoid costly missteps during implementation. This methodology became a foundation for every technical decision made over the following decade. It enabled the team to prioritise interventions based on measurable return, not assumptions. By using real operational data, the team could better understand interdependencies across the plant, an essential step in moving from partial gains to full energy self-sufficiency. Rather than relying on isolated upgrades, Jacobs and VCS committed to holistic, long-term improvements.

These measures focused on process changes to reduce energy consumption and increase power generation. One of the first steps involved re-routing organic carbon in wastewater from early treatment stages to anaerobic digesters, producing methane-rich biogas while reducing power demand in subsequent treatment. This renewable energy powers a combined heat and power system that delivers electricity to the facility and heats local homes and buildings via a distribution network spanning more than 20 kilometres from Ejby Mølle. These changes improved energy efficiency, reduced operational risk and created greater flexibility in plant performance.

VCS also piloted a full-scale membrane aerated biofilm reactor (MABR) system – one of the first in Europe. This technique diffuses oxygen directly through membranes into biofilms, significantly cutting energy costs and facility footprint. To enhance capacity while reducing energy use, Jacobs recommended granular deammonification, a biological process that supports efficient ammonia removal.

Supplying the grid with renewable energy

Ejby Mølle achieved energy neutrality in 2013, two years ahead of target, with a capital investment of just $2 million. VCS and Jacobs have also overseen the facility’s transition from a major electricity consumer to a power self-generator. By 2015, Ejby Mølle was producing 150% of its energy requirements. The excess electricity is sold to the local grid, with its thermal energy distributed to homes and commercial buildings across Odense. Efforts by VCS didn’t end there: Ejby Mølle now also recovers thermal energy from treated effluent, owned and operated by sister utility Fjernvarme Fyn. This additional recovery work saw the plant reach an astounding 330% net energy self-sufficiency in 2024. In practice, this means the plant produces more than three times the energy it consumes, transforming a traditional treatment plant into a net contributor of renewable energy.

Importantly, the project meets or exceeds all environmental regulations. The plant consistently achieves nitrogen concentrations below 6.0 mg/L and phosphorus below 0.5 mg/L, without needing to import external carbon sources. This compliance is maintained year-round, despite seasonal and load variability, highlighting the resilience of the upgraded system. Energy, emissions and effluent targets have been met without compromising on cost-effectiveness, safety, or operational reliability. The plant’s performance shows that ambitious environmental goals can be met within realistic operational budgets when data and collaboration are at the core.

A blueprint for wastewater utilities

The transformation of Ejby Mølle shows that legacy facilities can lead the way in the clean energy transition. So much so that Ejby Mølle has received global recognition, including a Silver Award in the International Water Association’s Project Innovation Awards and Water Project of the Year in the 2025 Sustainability Delivery Awards. The project creates a benchmark for other utilities looking to decarbonise. It shows that instead of being a waste byproduct that needs to be treated, wastewater can become a valuable source of renewable energy.

Ejby Mølle is also a testament to what happens when strong partnerships lean into innovative processes to realise ambitious goals. The result is a model for clean energy that can help to power the cities of the future. As the global water sector looks for ways to reduce climate impact, Ejby Mølle offers a demonstrated, operational model that can be scaled and adapted in cities around the world.